This invention relates generally to the use of solventless extrusion coating techniques for forming high transparency protective films and multi-layer paint coated films and laminates. More particularly, coatings are made by extrusion coating one or more layers onto a carrier sheet to produce films of high optical quality at high speeds while avoiding solvent emission problems characteristic of the use of solvent-based coatings. Techniques are also disclosed for removing multiple sources of defects from the resin manufacturing, handling and extrusion process, with the result that extruded clear films can be produced with an essentially defect-free glass-like clarity.
The present invention is described below with respect to its application to the manufacture of exterior automotive body panels, although other end-uses of the films made by this invention also are considered to be within the scope of this invention.
Exterior automotive body panels have been made in the past by spray painting sheet metal parts. Multi-layer paint coats, such as those referred to as a clear coat/color coat paint finish, have been used to produce desirable optical effects. In addition to high gloss and high distinctness-of-image (DOI), these paint coats also are highly durable by providing chemical resistance, abrasion resistance and weatherability that significantly reduces degradation by ultraviolet light.
In more recent years molded plastic car body panels have been made with decorative clear coat/color coat paint films bonded to the molded plastic panel. Use of such films avoids certain environmental problems associated with evaporation of paint solvents while also reducing or eliminating the need for paint facilities and emission controls at the automotive production plant.
Because of the growing need to reduce the amount of atmospheric pollution caused by solvents emitted during the painting process, many different approaches have been taken in recent years for producing these decorative films. These processes are generally categorized by solution casting techniques or extrusion techniques. For instance, U.S. Pat. No. 4,810,540 to Ellison et al. and U.S. Pat. No. 4,902,557 to Rohrbacher use solution casting techniques in which liquid-cast, solvent-based clear coats and pigmented base coats are applied to a flexible casting sheet by a coating process such as reverse roll coating or gravure printing. The liquid cast layers are separately applied and then dried at high temperatures to evaporate the solvents.
As an alternative, extruded films have been used for making exterior automotive clear coat/color coat films. International Application PCT US93 07097 to Duhme describes a process in which an injection molded laminate is made from an extruded clear coat layer, a color coat layer, a reinforcing layer laminated to the color coat layer, a bonding layer on a side of the reinforcing layer opposite from the color coat, and an injection molded substrate bonded to the bonding layer. The outer clear coat layer is a coextruded sheet having different proportions of polyvinylidene fluoride (PVDF) and acrylic resins in each layer of the coextrusion. An extruded thermoplastic liner layer is laminated to the outer surface of the clear coat layer to assist in injection molding the paint film laminate to the substrate. The coextruded outer clear coat layer is laminated to a polyester carrier which supports the clear coat layer during subsequent lamination steps. The outer clear coat layer can optionally be extruded onto the thermoplastic liner layer to provide gloss control. The color coat is made by solvent casting it on a carrier and laminating the dried paint coat to the clear coat. The reinforcing layer is laminated to the exposed side of the color coat, and the bonding layer may be coated on or laminated to the reinforcing layer. This process involves time-consuming multiple coating and lamination steps and slow processing speeds disclosed in the various examples.
U.S. Pat. Nos. 4,317,860 and 4,364,886 to Strassel also disclose coextrusion of multi-layer films such as a two-layer coextrusion of predominantly PVDF on one side and a predominantly acrylic resin on the other side of the coextruded sheet. These unitary structures are used to make molded articles, or to adhere the sheets to a molded polymer.
Film extrusion techniques also have been used in the past for making free films in which the extruded polymeric material is coated on a polished drum. These films are then undercoated with various color coats. The exterior surface of the extruded free film that contacts the drum (and is separated from the drum as a free film) does not have a high gloss and high distinctness-of-image. Also films manufactured in this manner do not have a carrier sheet attached, which makes them hard to handle and easily damaged in subsequent processing.
Another process disclosed in U.S. Pat. No. 5,114,789 to Reafler comprises a pigmented base coat which is solvent-die extrusion coated onto a flexible, stretchable carrier sheet and dried at elevated temperatures to evaporate the solvents, followed by extrusion coating a reactive clear coat on the base coat. The carrier film and extrusion coated paint layers are then heat softened as a unitary sheet and applied to a molded shaped substrate by a shrink wrap process.
In a currently used process for making exterior automotive paint films, a clear coat and color coat comprising blends of PVDF and acrylic resins are cast by reverse roll coater, either by solution or dispersion casting. The film thickness of the paint coats used in the process generally is dictated by end user requirements. In some instances the need to produce relatively thick films can impose certain production constraints. To adequately dry the material and to prevent air entrapment, line speeds are typically at 25 feet per minute. This slow throughput limits the coating capacity of the reverse roll coater and also releases a large amount of organic solvents. This solvent release is particularly evident when a solution-cast PVDF/acrylic clear coat is coated from a solvent-based solution having a relatively high amount of solvent. VOC emissions are high. PVDF has limited solubility and requires strong solvents to dissolve. One such solvent known as N-methyl pyrrolidone (trade name M-Pyrol) is either needed to solubilize the resin in solution casting or used as a coalescing aid in dispersion casting. In addition, cross contamination can occur from solubilizing residual material in previously used drums, hoses, pans, pumps, etc. Also, during coating, the strong solvent can dissolve caked-on resins in a drying oven, causing them to cascade down on the web being coated. As a further concern, these strong solvents are expensive.
Thus, there is a need for producing decorative and protective surfacing films while avoiding the adverse effects of low production line speed, high VOC, cross-contamination, and the use of expensive solvents.
Extrusion techniques can be an alternative that avoids the use of strong solvents and their related solvent emission problems. Extrusion techniques such as those described above, however, have not been successfully adapted to producing high optical quality films at high line speeds and at low cost.
Application Ser. No. 08/793,836 to Enlow et al. describes a solventless extrusion coating process that provides an alternative to both solvent casting and conventional extrusion of polymeric films. Use of the extrusion coating techniques of that invention provide the advantages of avoiding expensive solvents, producing no VOC emissions, and avoiding cross-contamination associated with solvent casting. In addition, the invention has the added advantages of greatly increasing line speed, eliminating steps in the manufacturing process, and reducing the cost of producing clear coat/color coat films. The invention has particular applicability to the manufacture of molded plastic exterior automotive body panels and parts, in that it provides a means for producing extruded high gloss, high DOI (distinctness-of-image) clear coat films of exterior automotive quality.
It has been recognized that solventless extrusion of polymeric materials into highly transparent, essentially defect-free thin film layers is extremely difficult. When such films are extruded for the purpose of providing a high gloss protective outer clear coat layer for an automotive laminate, for example, the layer is typically extruded as a thin film approximately one mil to three mils thick. However, the human eye catches the slightest defects in such a thin outer clear coat layer of high gloss and high DOI when compared with thicker films extruded as sheets or films that do not have the requirements of high gloss and high DOI.
It has also been recognized that even when a high gloss outer clear coat film is extruded as an essentially defect-free film, the film itself can replicate defects present in an underlying laminate to which it is bonded. For example, in an automotive laminate having an extruded polymeric backing sheet and size coat layer, defects can be telegraphed to the surface of a thin protective outer clear coat layer of high gloss. In this instance, defects as small in size as 10 microns or less in the extruded sub-layers can appear as noticeable defects in the high gloss outer clear coat layer.
Generally speaking, polymeric films which are solvent cast are more easily produced as defect-free clear coat films of high gloss and high DOI when compared with films made by solventless extrusion of polymeric materials. The difficulty arises when extruding engineering plastics as high gloss, high DOI clear coat films. The extrusion process by its nature generates defects in the extruded material and there are several sources of these defects, all of which must be addressed in order to ensure the optical clarity and smoothness of the finished extruded film. For example, application Ser. No. 08/793,836 to Enlow et al. describes how high shear and heat generation in an extruded material can cause induced haze and gel formation and resultant optical defects or reduced optical clarity in the extruded film. That publication also describes how reducing heat histories (minimizing heat rise) when compounding PVDF, acrylic and UV stabilizer starting materials can improve the quality of films made from those materials. Modifications to the extrusion process in order to avoid such problems, however, should not adversely affect subsequent thermoforming operations or unreasonably reduce line speed during the production process.
The formulation of the starting material also can affect optical clarity. For instance, an optically clear film made from a blend of PVDF and acrylic resins can be extruded more haze free when the PVDF component of the starting material is reduced from a level of 70% to below about 65%.
Although the effects of gel formation and induced haze are minimized by the processing techniques described above, it has been discovered that use of these processing controls may not categorically produce extruded clear films of extremely high transparency ,free of defects because additional defects can be introduced from other sources.
The present invention is based in part on a recognition that film quality of a solventless extruded clear film can be adversely affected by airborne particulate substances that may enter the extrusion process from a variety of sources. Failure to remove these contaminants from the process can result in noticeable defects in a thin extruded high gloss clear film. These defects can adversely affect the finished product whether they are present in the extruded outer clear coat film or in an underlying size coat and/or substrate panel to which the protective clear film is bonded.
It has been discovered that micron-size airborne contaminants from various sources can pass through the extrusion process and end up creating optical defects in the finished product. For instance, dust particles 10 microns in diameter or less produce noticeable defects in an extruded transparent one mil thick high gloss film. Such defects from airborne contaminants also may not appear until the finished laminate is thermoformed which can cause the defects to appear at the surface. Such airborne contaminants can include not only dirt particles from the air but also fiberglass particles and polymer dust present in the production plant. These contaminants can be introduced into the extrusion process when the resinous starting materials are handled before or after film extrusion.
In addition, contaminants may be present in the resinous starting materials. Such contaminants may include glass fibers, carbon, metal bits and gels introduced from the resin manufacturing process.
Thus, a process for solventless extrusion of thin high gloss clear coat films must address the problems of: (1) avoiding gel formation and induced haze; (2) avoiding defects being introduced not only in an extruded outer clear coat film but also in underlying extruded substrate layers; (3) avoiding film handling problems while maintaining high production line speed; (4) avoiding introduction of contaminants from the starting materials and during the resin handling and extrusion process; and (5) providing a finished laminate that maintains high gloss and high DOI after the finished part is subjected to thermoforming temperatures and resultant elongation.
Although the invention is described above with respect to exterior automotive applications, the invention also has applicability as a protective and decorative coating for other articles such as interior automotive components, exterior siding panels and related outdoor construction products, marine products, signage, window glass and other interior or exterior film products. Vinyl (PVC) siding panels are an example of one use of the invention for producing outdoor weatherable decorative surfaces on extruded plastic sheets. The invention, however, is applicable to plastic substrate panels other than vinyl, such as polycarbonate, for example. The invention is particularly applicable to protective films having a requirement of high transparency free of optical defects, i.e., any protective film that would have glass-like optical properties.
The present invention provides a process for solventless extrusion of engineering resins to form highly transparent glass-like weatherable optically clear films essentially free of optical defects. The invention avoids introduction of defects from gel formation; avoids induced haze that reduces transparency; avoids defects present not only in an outer clear coat but also in an underlying coextruded bonding layer and supporting substrate panel; promotes material handling at high production speeds; avoids introducing airborne contaminants and other defects throughout the process that would otherwise cause micron size optical defects in thin high gloss extruded outer clear films; and produces thermoformable laminates that maintain high gloss and high DOI sufficient for exterior automotive use, as one example.
Briefly, one embodiment of this invention comprises a process for making a protective and decorative surfacing film comprising extrusion coating a solventless polymeric material from an extruder die directly onto a moving carrier sheet to form an extruded coating of uniform film thickness on the carrier sheet. The carrier sheet is preferably a high gloss, heat-resistant inelastic polymeric casting sheet. The extrusion coated layer is preferably formed as an optically clear first layer on the carrier which travels at high speed past the extruder die opening. The extrusion coated first layer is immediately hardened by a temperature reduction, such as by contact with chill roll, followed by applying a pigmented second layer in thin film form on the hardened first layer, to form a composite paint coat. In one embodiment this composite paint coat is laminated to a bonding layer coextruded with a supporting substrate sheet or panel. The carrier sheet is separated from the resulting laminate to expose an outer surface of the extrusion coated first layer as a high gloss surface with a high distinctness-of-image.
Another embodiment of the invention provides a process for the extrusion of high gloss, high transparency clear films from a particulate resinous starting material essentially free of airborne contaminants, comprising holding the resinous starting material in a container, withdrawing the resinous material from the container and passing at least a portion of the resinous material through a dryer, and transporting the dried resinous material to an extrusion apparatus. The resinous material is conveyed from the container through the dryer and to the extruder in a closed air flow transport system in which the resin transport air is subjected to high efficiency (HEPA) filtration to remove micron size contaminants (defined herein as particles lower than about 10 microns in diameter) from the airflow that transports the resinous material. The resinous material is extruded as a transparent film essentially free of micron size defects.
The system for removing airborne contaminants includes a closed airflow conveying system subjected to high efficiency (HEPA) air filtration for transporting the resinous materials (1) to the extruder, (2) to and from a blending apparatus when used for blending multiple resinous materials prior to extrusion, and (3) to and from the dryer for removing any moisture from the extrudable resinous materials. In addition to filtering transport air in the closed resin transport system, the invention also removes airborne particles from production equipment with which the extruded film comes in contact. This includes removal of airborne particles attracted to the carrier sheet web by static electric charges and steps for cleaning adherent particles from surfaces of the traveling carrier sheet before and after the extrusion step.
Such high efficiency (HEPA) air filtration is preferably adapted to remove any airborne particulate matter below five microns in diameter, and more preferably below one micron in diameter, from the resin handling and extrusion process.
Although various polymeric film-forming materials can be used for forming the extrusion coated outer layer, the preferred extrudable material is a blend or alloy of a fluoropolymer and an acrylic resin in which the fluoropolymer is preferably polyvinylidene fluoride (PVDF).
The pigmented second layer, in one embodiment, can be solvent cast onto the extrusion-coated first layer, or alternatively, the first and second layers can be formed as a coextrusion which is then coated onto the moving carrier sheet.
Other forms of the invention include coextruding various layers of the composite laminate including not only the clear coat and underlying color coat but also the size coat, tie coat and other functional coats as well, including the backing sheet or other substrate panel, sheet or film. The carrier can also be extruded in tandem with the other layers of the laminate. The HEPA filtration techniques for removing airborne particles from the resins are applicable to the extrusion of each of these component layers and their starting materials.
Since one or more layers of the composite paint coat can be extrusion coated using solid (solventless) polymers, the process avoids the use of expensive solvents and also avoids VOC emissions and cross-contaminations associated with solvent casting. The process also can reduce production time and costs. A line speed for extrusion coating can be at least 50 feet per minute and more commonly in excess of 200 feet per minute, as compared to 25 feet per minute for solvent casting techniques. In one embodiment, extrusion coating is carried out at a line speed in excess of 300 feet per minute and can be operated at a line speed approaching 380 feet per minute.
Such improvements in line speed and related improvements in quality of the extrudate are produced by controlling the compatibility of the blended polymeric materials that comprise the backbone of the extruded material. By matching the melt viscosities of the blended polymeric materials in that they are reasonably close to each other, the flow characteristics of the alloyed material when heated to the extrusion temperature produce a smooth, more uniform flow which also avoids stress formation and visual defects in the hardened film. The processing techniques for melt blending the starting materials and for extrusion coating the resultant film are especially useful when preparing transparent films from alloys of PVDF and acrylic resins.
These techniques when combined with the HEPA filtration removal of airborne particles produce films and laminates of exceedingly high optical clarity.
These and other aspects of the invention will be more fully understood by referring to the following detailed description and the accompanying drawings.